Lua was originally designed in 1993 as a language for extending software applications to meet the increasing demand for customization at the time. It provided the basic facilities of most procedural programming languages, but more complicated or domain-specific features were not included; rather, it included mechanisms for extending the language, allowing programmers to implement such features. As Lua was intended to be a general embeddable extension language, the designers of Lua focused on improving its speed, portability, extensibility, and ease-of-use in development.

From 1977 until 1992, Brazil had a policy of strong trade barriers (called a market reserve) for computer hardware and software. In that atmosphere, Tecgraf's clients could not afford, either politically or financially, to buy customized software from abroad. Those reasons led Tecgraf to implement the basic tools it needed from scratch.[5]

Lua's predecessors were the data-description/configuration languages SOL (Simple Object Language) and DEL (data-entry language).[6] They had been independently developed at Tecgraf in 1992–1993 to add some flexibility into two different projects (both were interactive graphical programs for engineering applications at Petrobras company). There was a lack of any flow-control structures in SOL and DEL, and Petrobras felt a growing need to add full programming power to them.

In 1993, the only real contender was Tcl, which had been explicitly designed to be embedded into applications. However, Tcl had unfamiliar syntax, did not offer good support for data description, and ran only on Unix platforms. We did not consider LISP or Scheme because of their unfriendly syntax. Python was still in its infancy. In the free, do-it-yourself atmosphere that then reigned in Tecgraf, it was quite natural that we should try to develop our own scripting language ... Because many potential users of the language were not professional programmers, the language should avoid cryptic syntax and semantics. The implementation of the new language should be highly portable, because Tecgraf's clients had a very diverse collection of computer platforms. Finally, since we expected that other Tecgraf products would also need to embed a scripting language, the new language should follow the example of SOL and be provided as a library with a C API.

Lua 1.0 was designed in such a way that its object constructors, being then slightly different from the current light and flexible style, incorporated the data-description syntax of SOL (hence the name Lua: Sol is also the Portuguese word for "Sun", Lua being the word for "Moon"). Lua syntax for control structures was mostly borrowed from Modula (if, while, repeat/until), but also had taken influence from CLU (multiple assignments and multiple returns from function calls, as a simpler alternative to reference parameters or explicit pointers), C++ ("neat idea of allowing a local variable to be declared only where we need it"[5]), SNOBOL and AWK (associative arrays). In an article published in Dr. Dobb's Journal, Lua's creators also state that LISP and Scheme with their single, ubiquitous data-structure mechanism (the list) were a major influence on their decision to develop the table as the primary data structure of Lua.[7]

In general, Lua strives to provide simple, flexible meta-features that can be extended as needed, rather than supply a feature-set specific to one programming paradigm. As a result, the base language is light—the full reference interpreter is only about 180 kB compiled[3]—and easily adaptable to a broad range of applications.

A comment in Lua starts with a double-hyphen and runs to the end of the line, similar to that of Ada, Eiffel, Haskell, SQL and VHDL. Multi-line strings & comments are adorned with double square brackets.

--condition = truewhileconditiondo--statementsendrepeat--statementsuntilconditionfori=first,last,deltado--delta may be negative, allowing the for loop to count down or up--statements--example: print(i)end

The generic for loop:

forkey,valueinpairs(_G)doprint(key,value)end

would iterate over the table _G using the standard iterator function pairs, until it returns nil.

Lua's treatment of functions as first-class values is shown in the following example, where the print function's behavior is modified:

dolocaloldprint=print-- Store current print function as oldprintfunctionprint(s)--[[ Redefine print function. The usual print function can still be used through oldprint. The new one has only one argument.]]oldprint(s=="foo"and"bar"ors)endend

Any future calls to print will now be routed through the new function, and because of Lua's lexical scoping, the old print function will only be accessible by the new, modified print.

functionaddto(x)-- Return a new function that adds x to the argumentreturnfunction(y)--[=[ When we refer to the variable x, which is outside the current scope and whose lifetime would be shorter than that of this anonymous function, Lua creates a closure.]=]returnx+yendendfourplus=addto(4)print(fourplus(3))-- Prints 7--This can also be achieved by calling the function in the following way:print(addto(4)(3))--[[ This is because we are calling the returned function from `addto(4)' with the argument `3' directly. This also helps to reduce data cost and up performance if being called iteratively.]]

A new closure for the variable x is created every time addto is called, so that each new anonymous function returned will always access its own x parameter. The closure is managed by Lua's garbage collector, just like any other object.

Tables are the most important data structures (and, by design, the only built-in composite data type) in Lua and are the foundation of all user-created types. They are conceptually similar to associative arrays in PHP, dictionaries in Python and hashes in Ruby or Perl.

A table is a collection of key and data pairs, where the data is referenced by key; in other words, it is a hashed heterogeneous associative array.

a_table={x=10}-- Creates a new table, with one entry mapping "x" to the number 10.print(a_table["x"])-- Prints the value associated with the string key, in this case 10.b_table=a_tableb_table["x"]=20-- The value in the table has been changed to 20.print(b_table["x"])-- Prints 20.print(a_table["x"])-- Also prints 20, because a_table and b_table both refer to the same table.

A table is often used as structure (or record) by using strings as keys. Because such use is very common, Lua features a special syntax for accessing such fields.[9]

point={x=10,y=20}-- Create new tableprint(point["x"])-- Prints 10print(point.x)-- Has exactly the same meaning as line above. The easier-to-read-- dot notation is just syntactic sugar.

By using a table to store related functions, it can act as a namespace.

Tables are automatically assigned a numerical key, enabling them to be used as an array data type. The first automatic index is 1 rather than 0 as it is for many other programming languages (though an explicit index of 0 is allowed).

A numeric key 1 is considered distinct from a string key "1".

array={"a","b","c","d"}-- Indices are assigned automatically.print(array[2])-- Prints "b". Automatic indexing in Lua starts at 1.print(#array)-- Prints 4. # is the length operator for tables and strings.array[0]="z"-- Zero is a legal index.print(#array)-- Still prints 4, as Lua arrays are 1-based.

The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero. For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value. If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).[10]

Extensible semantics is a key feature of Lua, and the metatable concept allows Lua's tables to be customized in powerful ways. The following example demonstrates an "infinite" table. For any n, fibs[n] will give the n-th Fibonacci number using dynamic programming and memoization.

fibs={1,1}-- Initial values for fibs[1] and fibs[2].setmetatable(fibs,{__index=function(values,n)--[[__index is a function predefined by Lua, it is called if key "n" does not exist.]]values[n]=values[n-1]+values[n-2]-- Calculate and memoize fibs[n].returnvalues[n]end})

Although Lua does not have a built-in concept of classes, object-oriented programming can be achieved using two language features: first-class functions and tables. By placing functions and related data into a table, an object is formed. Inheritance (both single and multiple) can be implemented using the metatable mechanism, telling the object to look up nonexistent methods and fields in parent object(s).

There is no such concept as "class" with these techniques; rather, prototypes are used, as in the programming languages Self or JavaScript. New objects are created either with a factory method (that constructs new objects from scratch) or by cloning an existing object.

Lua provides some syntactic sugar to facilitate object orientation. To declare member functions inside a prototype table, one can use function table:func(args), which is equivalent to function table.func(self, args). Calling class methods also makes use of the colon: object:func(args) is equivalent to object.func(object, args).

localVector={}Vector.__index=VectorfunctionVector:new(x,y,z)-- The constructorreturnsetmetatable({x=x,y=y,z=z},Vector)endfunctionVector:magnitude()-- Another method-- Reference the implicit object using selfreturnmath.sqrt(self.x^2+self.y^2+self.z^2)endlocalvec=Vector:new(0,1,0)-- Create a vectorprint(vec:magnitude())-- Call a method (output: 1)print(vec.x)-- Access a member variable (output: 0)

Lua programs are not interpreted directly from the textual Lua file, but are compiled into bytecode, which is then run on the Lua virtual machine. The compilation process is typically invisible to the user and is performed during run-time, but it can be done offline in order to increase loading performance or reduce the memory footprint of the host environment by leaving out the compiler. Lua bytecode can also be produced and executed from within Lua, using the dump function from the string library and the load/loadstring/loadfile functions. Lua version 5.3.4 is implemented in approximately 24,000 lines of C code.[2][3]

Like most CPUs, and unlike most virtual machines (which are stack-based), the Lua VM is register-based, and therefore more closely resembles an actual hardware design. The register architecture both avoids excessive copying of values and reduces the total number of instructions per function. The virtual machine of Lua 5 is one of the first register-based pure VMs to have a wide use.[12]Perl's Parrot and Android's Dalvik are two other well-known register-based VMs.

This example is the bytecode listing of the factorial function defined above (as shown by the luac 5.1 compiler):[13]

Lua is intended to be embedded into other applications, and provides a CAPI for this purpose. The API is divided into two parts: the Lua core and the Lua auxiliary library.[14] The Lua API's design eliminates the need for manual reference management in C code, unlike Python's API. The API, like the language, is minimalistic. Advanced functionality is provided by the auxiliary library, which consists largely of preprocessormacros which assist with complex table operations.

The Lua C API is stack based. Lua provides functions to push and pop most simple C data types (integers, floats, etc.) to and from the stack, as well as functions for manipulating tables through the stack. The Lua stack is somewhat different from a traditional stack; the stack can be indexed directly, for example. Negative indices indicate offsets from the top of the stack. For example, −1 is the top (most recently pushed value), while positive indices indicate offsets from the bottom (oldest value). Marshalling data between C and Lua functions is also done using the stack. To call a Lua function, arguments are pushed onto the stack, and then the lua_call is used to call the actual function. When writing a C function to be directly called from Lua, the arguments are read from the stack.

Here is an example of calling a Lua function from C:

#include<stdio.h>#include<lua.h> //Lua main library (lua_*)#include<lauxlib.h> //Lua auxiliary library (luaL_*)intmain(void){//create a Lua statelua_State*L=luaL_newstate();//load and execute a stringif(luaL_dostring(L,"function foo (x,y) return x+y end")){lua_close(L);return-1;}//push value of global "foo" (the function defined above)//to the stack, followed by integers 5 and 3lua_getglobal(L,"foo");lua_pushinteger(L,5);lua_pushinteger(L,3);lua_call(L,2,1);//call a function with two arguments and one return valueprintf("Result: %d\n",lua_tointeger(L,-1));//print integer value of item at stack toplua_close(L);//close Lua statereturn0;}

Running this example gives:

$ cc -o example example.c -llua
$ ./example
Result: 8

The C API also provides some special tables, located at various "pseudo-indices" in the Lua stack. At LUA_GLOBALSINDEX prior to Lua 5.2[15] is the globals table, _G from within Lua, which is the main namespace. There is also a registry located at LUA_REGISTRYINDEX where C programs can store Lua values for later retrieval.

It is possible to write extension modules using the Lua API. Extension modules are shared objects which can be used to extend the functionality of the interpreter by providing native facilities to Lua scripts. From the Lua side, such a module appears as a namespace table holding its functions and variables. Lua scripts may load extension modules using require,[14] just like modules written in Lua itself. A growing collection of modules known as rocks are available through a package management system called LuaRocks,[16] in the spirit of CPAN, RubyGems and Python eggs. Prewritten Lua bindings exist for most popular programming languages, including other scripting languages.[17] For C++, there are a number of template-based approaches and some automatic binding generators.

In 2003, a poll conducted by GameDev.net showed Lua as the most popular scripting language for game programming.[19] On 12 January 2012, Lua was announced as a winner of the Front Line Award 2011 from the magazine Game Developer in the category Programming Tools.[20]

A large number of non-game applications also use Lua for extensibility, such as LuaTeX, an implementation of TeX type-setting language.